Measuring How Births Swell the Population Isn’t Child’s Play

A recent report said the U.S. birth rate has dipped to a record low level. But another measure of the nation’s fertility remains comfortably above its historic low. The mismatch shows that even in a country with comprehensive birth statistics, summarizing population trends is far from straightforward.

Last week, Pew Research Center said the birth rate last year fell to 63.2 per 1,000 women age 15 to 44. That’s the lowest level since at least 1920, the earliest year for which reliable data are available. The report made headlines and even spurred calls for Americans to get procreating lest they fall behind economically.

But the U.S. fertility rate, an estimate of how many children a woman will have in her lifetime, is well above record lows. According to the Population Reference Bureau, a Washington, D.C., research center, it fell in 2011 to just below 1.9 per woman, down from 2.12 in 2007-the highest in the last 40 years-but above a record low of 1.74 in 1976.

Demographers disagree on which measure is best for tracking births’ contribution to population growth. The distinction matters because birth trends are monitored closely. A record-low level of fertility could augur problems for future economic growth, while a slight drop from a 40-year high may have less serious implications-particularly because fertility often declines during economic slowdowns.

“Births are at a record low, but it’s a much more complex story,” said Brady Hamilton, a statistician at the Centers for Disease Control and Prevention’s National Center for Health Statistics. “The devil is in the details.”